FRCR: Physics Lectures Diagnostic Radiology

1. FRCR: Physics LecturesDiagnostic Radiology Lecture 4 Film-screen radiography Dr Tim Wood Clinical Scientist

2. Overview • Film-screen radiography • Processing • Intensifying screens and the film cassette • The characteristic curve and sensitivity • Image quality

3. The story so far… • We know how X-rays are made in the X-ray tube and how they interact with the patient • We know how we control the quality and intensity of the X-ray beam, and hence patient dose, with kVp, mAs, filtration and distance • We discussed the main descriptors of image quality • Contrast • Spatial Resolution • Noise • Discussed ways to improve contrast by minimising scatter and using contrast agents • Remember, there is always a balance between patient dose and image quality – fit for the clinical task!

4. Film-Screen Imaging • Traditionally, all X-ray image capture has been through X-ray film Emulsion Protective layer Adhesive layer Film base Emulsion

5. Film • Polyester film base gives mechanical strength to the film – does not react to X rays • Emulsion consists of silver halide grains (AgBr) • The image is formed by the reaction of AgBr grains to X-ray photons • The sensitivity of the film depends on number of grains • Must be evenly distribution • Typically each crystal is about 1 μm in size • larger grains = more sensitive (contrast), • smaller grain = better resolution • Adhesive layer ensures emulsion stays firmly attached to base • Protective layer prevents mechanical damage

6. Film • Film is actually much more sensitive to visible light and UV than it is to X-rays • Hence, use a fluorescent screen to convert X-ray photons to light photons • Enables lower patient dose! • A latent image is formed upon exposure, which cannot be seen unless the film undergoes chemical processing • Mobile silver ions are attracted to electrons liberated by light photons, forming a speck of silver metal on the surface

7. Processing • The invisible latent image is made visible by processing • There are three stages to this process; • Development • Fixing • Washing

8. Processing • First stage is development: • Film is immersed in an alkaline solution of a reducing agent (electron donor) • Reduces positive silver ions to metallic grain of silver (black specks) • Unexposed crystals are unaffected by the developer – bromide ions repel the electron donor molecules • However, given sufficient time, the developer will penetrate the unexposed crystals • The amount of background fog is dependent upon the time, strength and temperature of the developer

9. Processing • Second stage is fixing: • If the film is exposed to light after the first stage, the whole film becomes black • To ‘fix’ the film, unaffected grains are dissolved by an acid solution, leaving the X-ray image in the form of black silver specks • Final stage is washing: • The film is washed in water and dried with hot air • Inadequate washing would result in a brown/yellow film over time (from excess acid) and smell

10. Processing • Automatic processors use a roller system to transfer the film through the different solutions • Regular Quality Assurance of the processor is vital for producing good quality radiographs • Image is then viewed by transmission of light from a light box with uniform brightness • Dark = lots of X-rays • Light = relatively few X-rays e.g. through bone

11. Production of a Radiograph

12. Logarithms • A logarithm is an exponent – the exponent to which the base must be raised to produce a given number • 104 = 10x10x10x10 = 10,000 • = log1010000 = 4 • i.e., 4 is the logarithm of 10000 with base 10 • Seen in many applications • Richter earthquake scale • Sound level measurements (decibels = dB) • Optical Densities blackness on film (OD) • Written as log10x or if no base specified in physics texts as log x it is interpreted as the same

13. Properties of logs • log101 = 0 • log1010 = 1 • log10xy = log10x + log10y • log10x/y = log10x - log10y

14. Optical Density • Optical Density: the amount of blackening in the film • Defined as the log of the ratio of the intensities of the incident and transmitted light • log is used as the eyes response is logarithmic

15. Optical Density • Optical density can be measured with a densitometer • From the definition, if 1% of light is transmitted, D = 2.0 • If 10% is transmitted, D = 1.0 • The density of an area of interest on a properly exposure film should be about 1.0 • Lung field may be ~2.0 • Areas with D>3.0 too dark to see any detail on a standard light box

16. Contrast • Contrast is the difference in optical densities Contrast = OD1 – OD2 • High contrast - e.g. black and white • Low contrast – e.g. grey and grey!

17. Intensifying screens • Film is relatively insensitive to X-rays directly • Only about 2% of the X-rays would interact with the emulsion • Requires unacceptably high doses to give a diagnostic image • An intensifying screen is a phosphor sheet the same size as the film, which converts the X-rays to a pattern of light photons • The intensity of the light is proportional to the intensity of X-rays • The pattern of light is then captured by the film • One exception is intraoral dental radiography, where screens are not practical

18. Intensifying screens • Modern intensifying screens use rare earth materials, which emit light that is matched to the sensitivity of the film being used • Spectral match between the emission of the screen and the absorption in the film e.g. blue or green • K-edges clinically relevant (39-61 keV) • Rare earth screens used as they very efficient at converted absorbed X-ray energy into light • Results in a ‘faster’ (more sensitive) system • The sensitive emulsion of the film must be in close contact with the screen

19. Intensifying screens • General radiography film usually double coated with emulsion on each side of the base • The front screen absorbs ~1/3 of X-rays and ~1/2 light travels forward and is absorbed by front layer of emulsion • Rear screen absorbs ~1/2 of X-rays transmitted through the front and exposes the rear emulsion • ~2/3 of total X-ray fluence absorbed in screens • Mammography only uses a single screen to maximise spatial resolution (more on this later) • Screen materials chosen to have no phosphorescence (delayed fluorescence) to avoid ghost images

20. The film-cassette • Flat, light tight box with pressure pads to ensure film in good contact with the screen(s) mounted on the front (and back) • The tube side of the cassette is low atomic number material (Z~6) to minimise attenuation • Rear of cassette often lead backed to minimise back scatter (not in mammo)

21. The characteristic curve Optical density • Plotting OD against log exposure gives the Characteristic Curve of the X-ray film • Different types of film – subtle differences but all basically the same Saturation Linear region, gradient = gamma Solarisation Fog Log exposure

22. The characteristic curve • Depends on type of film, processing and storage • Fog: Background blackening due to manufacture and storage (undesirable) • Generally in the range 0.15-0.2 • Linear portion: useful part of the curve in which optical density (blackening) is proportional to the log of X-ray exposure • The gradient of the linear portion determines contrast in an image and patient exposures must lie within this region • Need to match this to the clinical task! • Hence, film suffers from a limited and fixed dynamic range

23. The characteristic curve Optical density • Gradient of linear region = Gamma,  = OD2 – OD1 log E2-log E1 • Gamma depends on • Emulsion • Size and distribution of grains • Film developing • Gamma ~ Contrast • Latitude = useful range of exposures Linear region Latitude Log exposure

24. The characteristic curve • Gamma and latitude are inversely related • High gamma = low latitude • Wide latitude (low gamma) for chests • High gamma (low latitude) for mammography • At doses above the shoulder region, the curve flattens off at D~3.5 • Saturation, whereby all silver bromide crystals have been converted to silver • At extremely high exposures density will begin to fall again due to solarization • Not relevant to radiography

25. Film Speed • Definition: 1 / ExposureB+F+1 • Reciprocal of Exposure to cause an OD of 1 above base plus fog • Speed of film = sensitivity = amount of radiationrequired to produce a radiograph of standard density • Speed shifts H-D curve left and right • Fast film requires less radiation (lower patient dose) • Speed is generally used as a relative term defined at a certain OD; one film may be faster than another at a certain point on the curve

26. Factors affecting speed • Size of grains – larger means faster • This is the main factor and conflicts with the need for small crystals to give good image sharpness. • Fast films are grainier but reduce patient dose • Thickness of emulsion • Double layers of emulsion give faster films • Radiosensitisers added • (X-ray energy)

27. Effect of developing conditions • Increasing developer temperature, concentration or time increases speed at the expense of fog • Developer conditions should be optimised for maximum gamma, and minimum fog • Automatic processor has temperature controls and time maintained by roller speed • Concentration is controlled by automatic replenishment of the chemicals

28. Film-screen sensitivity • Intensification factor • Each X-ray photon generates ~1000 light photons • Just under half of these will reach the film • ~100 light photons to create a latent image • Hence, more efficient process • Intensification factor is the ratio of air KERMA to produce D = 1 for film alone, to that with a screen • Intensification factor typically 30-100 • Speed class • Most common descriptor of sensitivity • Speed = 1000/K, where K is air KERMA (in μGy) to achieve D = 1 • Typically 400 speed (K = 2.5 μGy)

29. Image quality • Contrast • Contrast in film-screen radiography is due to both subject contrast, scatter and gamma • Remember, high gamma = high contrast = low latitude (and vice-versa) • Contrast is fixed for any given film and processing conditions • Image detail may be lost if contrast is too high as it may be lost in the saturated or fog regions • Hence, vital to match gamma to the clinical task • Ambient light conditions and viewing box uniformity may also impact on the subjective contrast presented to the Radiologist • Use a darkened room, mask off unused areas of lightbox, etc

30. Image quality • Screen-unsharpness • The film-screen system has inherent unsharpness additional to geometric, motion and absorption • Only partly due to finite size of the emulsion crystals • Most significant effect is due to spread of light from the point of X-ray absorption in the phosphor, to detection by the film • Depends on the point in the phosphor where the interaction occurs • Thicker phosphor layers more sensitive (absorb more X-rays), but result in more blurring – allow lower patient doses

31. Screen-unsharpness Object Phosphor Film

32. Screen unsharpness • Speed class should be chosen carefully to match the application • e.g. 400-speed (thick phosphor) for thick sections of the body (abdo/pelvis), • e.g. 100-150-speed (thin phosphor) for extremeties (require detail) • Also may have reflective layer on top of phosphor to increase sensitivity (reflect light photons back to the film) at the expense of resolution • Colour dyes to absorb light photons at wider angles (longer path lengths) – at the expense of sensitivity

33. Screen unsharpness • Crossover – light photons from the front screen may be absorbed by the rear emulsion (and vice-versa) • Crossover is a significant contributor to overall unsharpness • Reason for only using one screen in mammography where resolution is critical • Minimise screen-unsharpness by ensuring good contact between the screen and film • Poor contact may result from damage to the film cassette

34. Film-screen in clinical practice • Kilovoltage: Increased kV gives… • Increased penetration = lower patient dose • Increased exposure latitude = larger range of tissues displayed, BUT lower radiographic contrast • Reduction in mAs = shorter exposures = less motion blur • mAs • Correct mAs must be chosen to ensure the correct level of blackening on the film – avoid under or overexposing the film • Too much = saturation, too little = ‘thin’ image • Produce standard protocols that can be adapted for patient size

35. Exposure Control For an acceptable image, require a dose at the image receptor of about 3 μGy for film-screen radiography This is the exit dose from the patient after attenuation Entrance surface dose (ESD) is much higher than this; ~10 times greater than exit dose for PA chest ~100 times greater for skull ~1000 times greater for AP pelvis ~5000 times greater for lateral lumbar spine

36. Automatic Exposure Control (AEC) • Limited latitude of film makes it difficult to choose correct mAs – skill and experience of radiographer • Alternative is to use an AEC to terminate the exposure when enough dose has been delivered to the film • AEC is a thin radiation detector (ionisation chamber) behind the grid, but in front of the film (though in mammo it is behind to avoid imaging the chamber on the film) • Usually three chambers that can be operated together or individually

37. Automatic Exposure Control (AEC) • When a predetermined level of radiation is detected, the exposure terminates • Choice of chambers determined by clinical task • e.g. left and right for lungs in PA chest, but central if looking at spine • Also has a density control that can increase or decrease exposure where necessary • AEC limited to exposures in the Bucky system

38. Modern Day • Film is dying out • Across most (but not all) of the country film is no longer used for General X-ray imaging • Only mammography (breast imaging), where very high resolution specialist film is used • This Trust no longer uses film for mammography, and is on the verge of being fully digital…